In order to appreciate the uniqueness of the instant invention several terms used herein require definition.
A high speed surface is defined as terrain which has no obstacles with height that exceeds wheel travel.
A low spring rate means that a wheel is allowed to travel or be displaced upwardly to accommodate a sudden change in terrain with minimal change in vertical force. Since the change in vertical force is what causes vertical body motion, a low spring rate suspension system allows a vehicle to travel such terrain with very little input to chassis and driver. A further advantage of a low spring rate is a decrease in the rearward thrust that is generated as the wheel encounters a positive bump. This "give" or "easing" of the rearward thrust not only reduces driver fatigue but it decreases the power necessary to traverse rough terrain. A low spring rate also decreases the forward thrust required to propel a given wheel up the leading edge of a low-speed obstacle, thereby reducing the tractive effort required. Since less traction is required, the modular independent suspension can negotiate low-speed obstacles with steeper leading edges and under more slippery conditions.
Long wheel travel refers to the vertical motion of a wheel during travel and refers specifically to the extension of the extremes of terrains which are classified as "rough", thereby increasing the variety of surfaces which the modular independent suspension can cross at high speeds. An increase in long wheel travel also increases the height of obstacles which can be negotiated with all tires in contact with the ground. Maintenance of tire-ground contact increases mobility and combats vehicle roll-over.
A low-speed surface is terrain made up of obstacles with heights that exceed wheel travel.
High ground clearance refers to the height of an obstacle that can pass between the tires of a given axle.
A flexible frame refers to the basic structural framework of the vehicle and connotes the ability of the frame to torsionally deflect during operation. The existence of a flexible frame, in effect, increases the effective wheel travel while negotiating low-speed obstacles.
As the road system deteriorates in most Western nations and as the demand for efficient low cost off highway vehicles increases, the use of independent suspensions on off highway vehicles is of greater and greater importance. Although many independent suspension systems have been developed and gone into use, none appear to be entirely satisfactory for a wide range of operating environments. For example, few, if any, independent suspension systems are truly modular in the sense that they can be applied to steerable or non-steerable wheels, and driven or non-driven axles. Further, the current trend is to design stronger and more rigid frame structures to meet increasingly severe operating conditions and, experience has shown, this is technically questionable in many applications. And in few if any systems is there any significant degree of commonality of components from wheel to wheel. This latter feature is of increasing significance because unique constructions, wheel to wheel, increase initial cost, maintenance costs, inventory stocking, and skill levels required to keep fleets of heavy duty vehicles operating at high efficiency. Further, in many present independent suspensions operator fatigue is a serious concern. In many systems the wheel travel is so low that the operator is constantly applying and releasing the throttle and yet, even with frequent speed changes, the vertical forces imparted to the operator's cab is frequent during an eight-hour shift.